U.S. Pat. Nos. 4,274,420 and Re. 31,454 describe flexible medical electrodes composed of a flexible silver foil backing to which is applied a flexible substrate or matrix containing a hydrophilic hydrocolloid, a hydrating liquid e.g. glycerin and an electrolyte to render the flexible hydrated gel electrically conductive. It has also been proposed for example in U.S. Pat. Nos. 4,419,998 and 4,494,552 to provide a combination heart monitoring and defibrillation electrode for medical use that includes a metal backing composed of tin having a layer of stannous chloride sprayed onto one surface. Over the stannous chloride layer is applied a porous foam disc adapted to receive an electrically conductive medium such as a saline gel. These electrodes while generally effective have certain disadvantages. The need for a laminate of several layers of material increases production costs. Moreover, soluble substances can diffuse from one layer to another allowing chemical changes to take place during shipment, storage and use. In addition, the foam material adds to the weight and bulk of the electrode but does not assist in carrying electric current. Moreover, delamination is possible between layers which may under some circumstances interfere with the conduction of current from one layer to another. At the present time, most medical electrodes used for monitoring purposes which require application over extended periods of time include a conductive silver/silver chloride material connecting the conductive gel to the metal snap. Electrodes having tin foil backing have been used successfully in a large commercial scale only for diagnostic purposes which generally require application for only a few minutes time. One of the major objectives of the present invention is to find a way to construct an effective, commercially functional medical electrode with a tin foil backing coated with a hydrophilic matrix containing a hydrocolloid in a hydrated state that can be used both for diagnostic as well as for monitoring use over a time period of many hours and which during use will have performance characteristics not unlike the more expensive electrodes containing silver/silver chloride conductive material.
In the development work leading to the present invention, electrodes were prepared using a tin foil backing coated with a hydrophilic gel containing a tin salt such as tin chloride dissolved in the gel. Such electrodes while they were effective for some purposes did not have good shelf life for some applications. Moreover, premix solutions used in the manufacture of these gels, containing tin salts, were unstable and heterogeneous.
To understand the requirements of the present electrode, some of the problems of clinical use will be briefly reviewed. One of the desired objectives is to provide a non-silver/silver chloride containing electrode which can be used for monitoring applications. However, to be effective such an electrode must depolarize quickly after a defibrillation stimulus of approximately 200 volts is discontinued. A diagnostic electrode that remains polarized following defibrillation is ineffective in picking up EKG signals from the heart. Depolarization is particularly important since following defibrillation with an ineffective electrode, the heart monitor trace will flip out of view on the screen and will not come back because the electrode itself looks like a battery to the monitor. Thus the monitor can not be used to tell if a patient is recovering following defibrillation.
A major goal is to provide effective coupling at the interface between the electrically conductive gel and the metal foil layer. This will permit rapid depolarization of the electrode after defibrillation. If the coupling is satisfactory, the electrode will exhibit a relatively low and constant D.C. offset, i.e. one that is constant over time rather than rising over a period of time after it is applied to the skin. D.C. offset is a minute current produced by the electrochemical makeup of the electrode itself. To be satisfactory the electrode should have a D.C. offset no greater than about 100 millivolts.
In addition, to be satisfactory a high performance electrode should have relatively low impedance at low frequencies, especially for EEG applications. For example, if a tin foil backing is coated with a hydrated flexible hydrophilic gel matrix containing sodium chloride as an electrolyte, the impedance will rise rapidly when the electrode is subjected to an applied alternating current as the frequency of that current decreases (say 100 to 10 cycles cycles per second). By contrast, commercially acceptable silver/silver chloride electrodes exhibit an impedance which is substantially constant when subjected to an AC current over the same range of frequencies. An electrode will not exhibit the desired uniform impedance characteristics if its hydrophilic gel composition is incompatible or does not couple well to the metal foil backing.
The desired electrode should exhibit two effects; first, rapid depolarization of charged internal layers caused by defibrillation pulses and second, relatively low and constant DC offset as well as constant impedance with a fluctuating voltage applied at various frequencies. Low impedance is important because it will provide a lower noise to signal ratio.